The present invention relates to an engine knock sensor for sensing knocking occurring in an internal combustion engine. More particularly, the invention is concerned with an engine knock sensor for use in an anti-knocking device which is adapted to perform such a control as to delay the ignition timing upon sensing a knocking occurring in an internal combustion engine.
A typical conventional knock sensor has a vibration pickup for detecting vibration in an internal combustion engine, a full wave rectifier for making a full wave rectification of the output from the vibration pickup, an integrator for obtaining the mean value of the output from the full wave rectifier through integration of the output, an adder for adding a predetermined voltage to the output from the integrator thereby to produce and output a judgement reference signal, and a comparator adapted for comparing the output from the vibration pickup with the judgement reference signal.
In this knock sensor, therefore, the electric signal directly derived from the vibration pickup is compared with the judgement reference signal which is obtained from the vibration pickup indirectly through the full wave rectifier, integrator and adder, and a knocking signal representing the occurrence of the knocking is generated when the level of the reference signal is exceeded by the level of the electric signal directly derived from the vibration pickup. The level V of the voltage of the judgement reference level is given by the following formula (1), EQU V=Ka.sub.o +b.sub.o (1)
where, K represents a proportional constant, a.sub.o represents the level of electric signal and b.sub.o represents the level of the offset voltage which is added in the adder.
The peak value of the electric signal derived from the vibration pickup is increased in proportion to the increase of the engine speed and takes an extremely small level when the engine speed is low, as will be understood from FIG. 1. More specifically, in FIG. 1, a curve A shows the peak values of the output from the vibration pickup in relation to the engine speed as observed when there is no knocking taking place in the internal combustion engine, while curves B,C and D show the peak values as observed in the states of light, medium and heavy knocking taking place in the engine, respectively. Therefore, as an electric noise N, which is generated when the wiper is driven or when the headlight is put on, is picked up by, for example, the signal line between the vibration pickup and the full wave rectifier, the noise is superposed to the electric signal from the vibration pickup to make the electric output signal indistinguishable from the electric noise, particularly when the engine speed is low, resulting in a knock sensing failure or erroneous sensing of knocking occurring in the engine.
The erroneous sensing of knocking would be avoided if the judgement reference signal were selected to be sufficiently high as compared with the level of the electric noise. This, however, will impose a new problem of difficulty in the sensing of the engine knock of low level. It is also considerable to amplify the electric signal produced by the vibration pickup, in order to sense even the small level of engine knock. In such a case, however, the electric signal derived from the vibration pickup during medium- and high-speed operation of the engine will become excessively large to exceed the dynamic range of the engine knock sensor to make the sensing of the engine knock materially impossible particularly in the medium- and high-speed engine operation.
Another problem involved by the conventional engine knock sensor is that the engine knock sensor is required to have an impractically wide dynamic range in order to cover the entire range of engine operation including low-speed and high-speed engine operation, because the peak value of the electric signal derived from the vibration pickup is largely changed in response to a change in the engine speed.